A conventional dielectric filter is now explained by referring to FIG. 4 which shows a pillar-shaped quarter-wavelength dielectric body 1 provided with a plurality of through-holes (in this case, three holes) 2, 3 and 4, respectively, which connect the upper and lower surfaces of pillar-shaped dielectric body 1, electrodes 5 provided on outer and lower surfaces of dielectric body 1, and pillar-shaped insulators 8 and 9 in which lead wires 6 and 7 are incorporated integrally and inserted in holes 2 and 4.
With the above-explained construction of a conventional resonator, two quarter-wavelength coaxial resonators are constructed of segments including holes 2 and 4 which are regarded as inner conductors, and hole 3 which adjusts the magnetic field coupling between the two coaxial resonators. Lead wires 6 and 7 are capacitively coupled to electrodes coated on the internal surfaces of holes 2 and 4 through insulators 8 and 9, and lead electric signals in and out.
According to the construction of the filter of FIG. 4, three through-holes 2, 3, and 4 have to be provided within said dielectric body 1, and this means that (2n-1) holes have to be provided on the dielectric body in order to construct a filter having n-stages of resonators. However, this construction work requires a highly complicated and precise press molding of dielectric ceramics to prepare dielectric body 1. This process is nearly impossible to apply to a small dielectric body 1 because of the close distances between holes 2, 3, and 4.
Moreover, this process difficulty is enhanced when a larger diameter hole 3 is required to adjust the coupling between the resonators. This process is definitely disadvantageous for constructing miniature dielectric filters. Moreover, since two independent insulators 8 and 9 have be consistently provided, this increases the number of parts and the assembly difficulty.